In the drilling and production of oil, gas and other subterranean wells, it is often necessary to obtain at the surface, information concerning conditions within the borehole, well below the surface. For example, it is often desirable to confirm the operational condition of a piece of downhole production equipment or to determine the conditions of the downhole environment itself. Various techniques and equipment are known for use in obtaining, at the surface, information about the conditions within a borehole. One such approach has been to lower an inspection device, such as an optical sensor positioned on the end of a section of coiled tubing, into a borehole and transmit information received by the device beneath the surface to measurement equipment at the surface through the coiled tubing. Additionally, certain condition monitoring may be desirable during the drilling process itself, and/or during the operation of the well, such as monitoring of a submersible pump. Accordingly, drill heads and pumps are often outfitted with monitoring devices that are connected to the surface for the transmission of those signals to measurement equipment on the surface.
Fiber optic communication systems have become increasingly common in supplanting traditional wire-conductor communication systems in such downhole inspection devices and tools. The advent of optical fiber cable systems for use in such downhole devices and tools has created the related need for fittings, connectors, and feedthroughs capable of use with optical fiber systems. Optical fiber can be particularly vulnerable to environmental stresses and contamination.
Drilled wells can be extremely aggressive environments. The inner surface of boreholes tends to be very rough, and in addition, drilling muds, which are used to facilitate drilling, contain chemical additives which can degrade non-metallic materials. Such muds are often highly caustic with a pH as high as 12.5. Furthermore, other fluids that may be present in a borehole can include saltwater, crude oil, carbon dioxide, hydrogen sulfide and others, many of which are highly corrosive to a wide variety of materials. Downhole conditions become progressively more hostile and aggressive as the depth of the bore increases. For example, at depths of 5,000 to 8,000 meters, bottom hole temperatures can easily exceed 250° C., and pressures of well over 15,000 psi are often encountered. These types of high-pressure/high-temperature conditions can serve to amplify the corrosive and degrading nature of the downhole environment.
Such downhole conditions, including high pressure, high temperature and corrosiveness, can easily damage external or exposed downhole tool components. Internal electronics and optics need to be provided with protection from the elevated temperatures pressures and corrosive environment.
One area of particular concern regarding the potential for damage to optical fibers used in the downhole environment is the point at which an optical fiber passes from its protective outer coating (e.g., high strength metal sheathing) into the bulkhead, or external housing, of a downhole tool or device. In order for the optical fiber to serve its intended purpose and convey accurate optical signals from the downhole tool to the surface measurement instrument, all connections, seals fittings and feedthroughs associated with the fiber must be able to provide protection for the fiber from the environmental conditions without affecting its capacity to transmit unmodified optical signals. In addition, any such seals, fittings, etc. must not create stress on the optical fiber. Such external stress on an optical fiber can lead to improper transmission of the optical signal and/or physical failure of the fiber.
A number of methods for providing hermetic seals for optical fibers used in the downhole environment have been proposed. However, with the ever increasing demand for stronger, more accurate and wider bandwidth optical transmission, improved seals and connection assemblies for fiber optic cables are sought.